What Size Cable for Sockets and Other Home Circuits?
Updated: Oct 9
Most UK socket circuits are wired in 2.5 mm² twin and earth, either as a ring on a 32 A breaker or as a radial on a 20 A breaker. A radial socket circuit on a 32 A breaker typically steps up to 4 mm². Those are the conventional arrangements, but they are typical rather than fixed: your electrician will size any real circuit to the load and the installation method, because how the cable is installed and how far it runs can change the answer.
Typical cable sizes by circuit
These are the sizes an electrician would expect to see in a typical home. Treat them as a guide to what is normal, not a design.
Lighting: 1.0 mm² or 1.5 mm² twin and earth, usually on a 6 A breaker.
Ring final circuit (sockets): 2.5 mm² on a 30 A or 32 A breaker, serving a floor area of up to about 100 m².
Radial sockets, 20 A: 2.5 mm², up to about 50 m².
Radial sockets, 32 A: 4 mm², up to about 75 m².
Cooker: its own circuit, often 6 mm² for a typical household cooker, depending on the cooker's rating and the circuit length.
Electric shower: its own circuit in a heavier cable than any socket circuit, sized to the shower's kW rating and the length of the run.
EV charger: a dedicated circuit, typically 32 A for a 7 kW single-phase charger, sized by calculation for the route.
Why 2.5 mm² works on a 32 A ring
On its own, 2.5 mm² cable cannot safely carry 32 A in most installation methods. A ring gets round that because the load is shared between the two ends of the loop. BS 7671 regulation 433.1.204 gives 30 A and 32 A rings in 2.5 mm² cable "deemed to comply" status, and that is why the arrangement is so common.
The same cable used as a radial is normally limited to 20 A. If a 32 A radial is wanted, for example for a run of kitchen worktop sockets, it moves up to 4 mm². Planning a new socket circuit from scratch is part of our sockets and power installations work, and the cable size is set by calculation before a single cable goes in.
What 2.5 mm² cable is used for
Search for "2.5mm cable" and you are nearly always looking at 2.5 mm² twin and earth: two insulated conductors (brown and blue) plus a bare 1.5 mm² earth between them. In a home it is the workhorse for:
ring final circuits for sockets;
20 A radial socket circuits;
spurs off those circuits, which must match the circuit's cable size unless they are fused;
some fixed appliances such as immersion heaters, on a suitably rated radial.
The 4 mm² version also has a 1.5 mm² earth, as IET test examples for 4.0/1.5 mm² cable show. The earth conductor is always sleeved green/yellow where it is exposed at a terminal. If you meet older red and black conductors instead of brown and blue, our guide to live wire colours explains what you are looking at.
Why installation method and length change the answer
The same cable can carry more or less current depending on where it runs. The main factors are:
Thermal insulation. Cable buried in loft insulation or an insulated stud wall cannot shed heat. Electrical Safety First's regulations guidance shows a 2.5 mm² ring may need to drop to a 20 A breaker to cope with insulation.
Grouping. Several cables bundled together warm each other up, which lowers what each can carry.
Length. Long runs, such as to a detached garage or garden room, lose voltage along the way. The longer the run, the more likely a bigger cable is needed.
Earth fault protection. The circuit has to disconnect fast enough under a fault, which also depends on cable size and length.
This is why a sparky will not give you a firm answer over the phone for a long or unusual run. Cable to an outbuilding that goes underground also has to be armoured or run in a protective duct, which is covered in our post on outdoor sockets.
Shower cable size
Showers draw a lot of current for a long time, so they always have their own circuit. The cable size depends on the shower's kW rating, the breaker and the length and route of the cable, so there is no single right answer: a more powerful unit or a longer run needs a thicker cable, and your electrician will work it out for your installation.
The trap is upgrading to a more powerful shower on the old cable. A higher-rated unit draws more current, so the existing cable, the breaker or both may no longer be right. Check before you buy, not after it is on the wall. A new shower circuit is notifiable under Part P, and so is any alteration to wiring in the zone around the shower, so this is always a registered electrician's job.
Can you use 1.5 mm² cable for sockets?
Not for a normal socket circuit in twin and earth. 1.5 mm² is a lighting cable in most homes, and a socket circuit wired in it would be undersized for the breaker protecting it. If you find 1.5 mm² feeding sockets, have it checked.
SM Electrical sizes, installs and certificates socket, cooker and shower circuits for homes and businesses around Shifnal, Telford and Wolverhampton, including anyone looking for an electrician in Shrewsbury.
Cable size questions, answered
Can I extend a ring with 1.5 mm² cable to add one more socket?
No. An unfused spur has to be the same size as the circuit it comes from. If you want to use a smaller cable, it has to go through a fused connection unit first.
Is 4 mm² better than 2.5 mm² for a ring?
It is not needed for a standard ring, although 4 mm² is sometimes used where a ring runs through insulation or is bunched with other cables. Bigger cable is harder to fit into socket terminals and back boxes. Where a circuit needs more capacity, splitting it into radials is usually the better design.
Do I need a bigger cable for a longer run?
Often, yes. Voltage drop and earth fault disconnection times both get worse with length, so long runs are sized by calculation rather than by a table.
Does a new circuit need a new consumer unit?
Only if there is no spare way or the board lacks RCD protection. A new circuit is notifiable work in its own right, and sometimes it is the moment to consider a consumer unit upgrade.
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